Wavelength Division Multiplexing (WDM) is a technique used to transmit a plurality of optical channels via an optical waveguide medium where each channel carries information signals within a network or system. Each channel within the WDM signal is associated with a particular wavelength, thereby increasing the information capacity of fiber optic systems. WDM optical networks have traditionally been used for long haul point-to-point networks. However, with the increasing demands on communication systems, WDM optical networks can also be used in smaller system configurations, such as local telephone or data networks. In these systems, communication signals are usually transmitted over a limited geographic area to various nodes within a network, thereby avoiding the need for amplifiers. A particular node can be configured to drop one or more information bearing or payload channels from the WDM signal, process the information contained in the dropped channels and add channels containing new information to the WDM signal for transmission to other nodes in the network. An optical add/drop multiplexer present at each node may be used to drop the particular channel from the WDM signal and subsequently add the channel back to the WDM signal prior to transmission to another network node while allowing the remaining channels to passthrough.
Nodes within these types of networks can be separated by optical paths of differing lengths. In order for nodes to successfully receive WDM signals within the network, the power associated with the WDM signal transmitted between nodes varies depending upon the length of the optical path between the origination and destination nodes. For example, when a channel is added back to the WDM signal at a particular node within a network, the associated transmitter provides sufficient power to allow the signal to travel to the destination node and its associated receiver. However, if the destination node of a particular channel is in close proximity to the transmitting or source node, the transmitting node may provide too much power to the signal. This causes a problem because optical receivers have a corresponding "dynamic range" whereby an increase in the input optical power and the associated output electrical current have a linear or substantially linear relationship. If input power levels increase excessively, the receiver output current can saturate and no longer increase with corresponding increases in input optical power. As a result, the optical signal input to the receiver cannot be accurately detected.
Moreover, when a channel is dropped by an optical add/drop multiplexer, a portion of that channel signal may leak into the pass-through channels. When the dropped channel is subsequently added back to the pass-through channels, coherent crosstalk may occur between the added channel signal and the leaked portion of that channel. This crosstalk may be sufficient to cause signal recognition problems.
Thus, there is a need to control/fix the output power of node transmitters to correspond to the dynamic range of node receivers within a network. In addition, there is a need to reduce coherent crosstalk associated with particular channel transmissions.